As is well known, wood material to be refined is introduced into the eye of a refiner and moved through a refining zone between the refiner plates. This material is progressively broken down into smaller pieces and finally into individual fibers and fiber fragments in the gap between the relatively rotating refiner plates.
In operating a refiner, water is supplied with the wood material to provide a relatively accurate control of consistency within the refiner as the consistency has a significant bearing on refiner operation and thus on pulp properties.
The other major operating variables in a refining process are the rate of infeed of the wood material which is controlled by the speed of the infeed conveyor; and the energy applied, usually specified as specific energy and defined as the net power applied which is the difference between the total power and the backed off power (no load condition) divided by the pulp mass flow through the refiner. The applied power is maintained by changing the closing pressure which changes the gap between the refiner plates.
It will be apparent that if the gap between the refiner plates reduces to zero, the refiner plates will clash resulting in metal to metal contact and at least excessive wear of the plates.
It will also be apparent that over time the refiner plates which are formed with various patterns of lands and grooves wear and as a result, the operation of the refiner changes over time.
In operation, the operator usually sets the motor load setpoint (i.e. load to be applied by the refiner discs to the pulp pad) based on the required pulp properties and production rate for a selected consistency within the refiner. If this setpoint is too high, the operation of the refiner may be impaired as the tendency for pad collapse will frequently occur and the risk of plate clashing is significant.
As above indicated, plate clashing is avoided if possible and thus safety measures have been built into the control system of the refiner to cause the plates to separate when pad collapse appears to be eminent.
Thus, a good system of determining impending pad collapse is of significant value.
In practice, the relative position or spacing between the cooperating plates forming the refining zone is monitored and when the plates become too close, they are immediately backed off to avoid impending disaster. The more common system to accomplish this monitors vibrations of the equipment and if a preset threshold is exceeded, the plate gap is increased (opened) in large step wise increments and in some cases, the refiner shuts down. Shut down of the refiner is a relatively costly remedy.
Adaptive control systems have been proposed and implemented to control the power application to a refiner based on the relationship between the plate gap and motor load whereby the plates are moved apart when the sign of the slope of the curve of power versus plate gap changes. Such a system was first proposed by Guy Dumont in Automatica, Vol. 18, No. 3, pp 307-314, 1982, in a paper entitled "Self-tuning Control of a Chip Refiner Motor Load" and further discussed in a paper entitled "Control of a TMP Plant", G. Dumont et al., Pulp and Paper Canada 83:8 (1982), pp T224-T229.
In "Thermo Mechanical Pulping Process Control" by Jones and Pila presented at the Canadian Pulp and Paper Association Annual Meeting 1983, pp B105-B111 of the preprints, a hierarchical approach to the control of thermo mechanical pulping is discussed wherein the control of specific energy is described as one of the modular sub-systems of the overall control. This paper describes a system similar to that of Dumont et al. referred to above in that they propose the use of an estimate of the process gain (slope of the curve of plate gap versus power or motor load) and its sign based on the ratio of moving averages of incremental change in motor load to incremental change in plate gap. This moving average is then used to override the specific energy control and force the plates to separate if the sign of the process gain changes (i.e. moves from the stable to the unstable region) over the average selected number of samples.
A paper presented at the Mini and Micro Computer Conference in Saint Fielu, Spain, in June 1985 entitled "A Microprocessor-Based Control System for a TMP Refiner" by Koivo et al., describes an adaptive control system similar to that of Dumont described above, although very little information is given.
An article entitled "Wood Chip Refiner Control" by Dumont and .ANG.strom, IEEE Control Systems Magazine, April 1988, pp 38-43 inclusive, suggests another way of improving reliability of the adaptive controller first described by Dumont in his paper entitled "Self-Tuning Control of a Chip Refiner Motor Load". The improved system provided a substantial improvement over the previous method in that it allowed for smoother transition between the controlling and retracting modes. This system suggests actively probing the refiner to improve the accuracy of the gain estimate, and incorporates an indication of the accuracy of the gain estimate to improve the reliability of the method. This system has never been implemented on a refiner.
The paper entitled "Adaptive Control Using a Dahlin Controller with Application to Wood Chip Refining" by Banerjee et al. describes the adaptive control system similar to that of Dumont described above. This system has never been implemented on a refiner.
Kooi et al. in a paper entitled "Control of Wood Chip Refiner Using Neural Networks" published in a TAPPI Journal of June 1992, pp 156-162 inclusive describes a neural network-based controller as an alternative means to overcome some of the shortcomings of the adaptive controls schemes for chip refiners and is based on the fact that the neural network does not require exact mathematical description of the process it is controlling. This system has never been implemented on a refiner.
All of the methods just discussed are based on characterizing the motor load plate gap relationship with a linear model. This has some limitations. First, a linear model is monotonic and, therefore, cannot characterize the motor load peak. By extrapolation, the controller "thinks" that there is no limit to the maximum achievable load. Secondly, because a motor load which is less than the peak load can be achieved with two values of the plate gap, one of which ultimately leads to a pad collapse, the controller must be modified to keep it from attempting to regulate the load in the pad collapse region. Dumont and Fu in a paper entitled "Nonlinear Adaptive Control via Laguerre Expansion of Volterra Kernels", presented at the 2nd Workshop on Adaptive Control: Applications to Nonlinear Systems and Robotics, Cancun, Mexico, December 1992, proposed an approach based on a nonlinear model. An advantage of this is that the motor load peak and the input multiplicity are easily dealt with. That is, when the setpoint is less than the maximum, there are two possible inputs. The one corresponding to the larger plate gap is implemented. When the setpoint is greater than the maximum, there is only one possible input. This corresponds to the peak load. This method requires more development before it can be applied to an industrial refiner.